Electronic engine control systems are commonly employed to control the operation of internal combustion engines. By precisely controlling engine functions such as air-fuel ratio, EGR and ignition timing, electronic engine control systems can optimize such variables as fuel economy, performance and exhaust emissions. To effectively control engine operation, a number of conditions can be sensed while the engine is in operation. These conditions may include crankshaft position, air/fuel ratio, engine speed, exhaust emissions, etc. Open-loop control systems utilize engine variables such as engine revolutions per minute and load to calculate engine control parameters. However, open-loop control is only correct for special conditions and does not take into account such variables as combustion chamber deposit formation, fuel composition, engine wear, etc.
To overcome the deficiencies inherent in open-loop systems, closed-loop systems, utilizing direct sensing of combustion processes for feedback control are being developed. With direct sensing of combustion processes, information such as the charge burn duration could be used to optimize such input parameters as spark advance and air/fuel ratio. This could improve engine efficiency by insuring an optimum average peak pressure arrival time.
Two ways to directly sense the combustion process are to use pressure sensors and ionization probes. A method of measuring burn duration utilizing pressure sensors is described in U.S. Pat. No. 4,736,724, issued to Hamburg, et al. However, pressure sensors are not considered practical for automotive applications because of limited durability, temperature instability and high cost. On the other hand, ionization probes offer a more durable and less costly alternative to pressure sensors. Ionization probes sense the presence of a flame at a particular location in the combustion chamber by sensing the flow of electrical current through the flame from the probe to the combustion chamber wall. The use of ionization probes for closed-loop control of internal combustion engines is described in applicant's U.S. Pat. No. 4,257,373, entitled "Internal Combustion Engine Ignition System". Considerable experimental work has been done on ionization probes subsequent to the issuance of U.S. Pat. No. 4,257,373. See, for example, S.A.E. Paper No. 840441 by Michael G. May, entitled "Flame Arrival Sensing Fast Response Double Closed Loop Engine Management", and S.A.E. Paper No. 860485 by Robert L. Anderson, entitled "In Cylinder Measurement of Combustion Characteristics Using Ionization Sensors". As reported in the latter S.A.E. Paper, it has been found that ion probes can provide an accurate indication of peak pressure time at stoichiometric air/fuel ratios. However, at leaner than stoichiometric ratios, such as those present in "lean burn" engines, the data from an ion probe has a large amount of cycle-to-cycle variance. Because of this variance, the usefulness of ion probes is marginal in automobile engine control systems.
It is believed that much of the large cycle-to-cycle variance in ion probe data is caused by flame front raggedness. At stoichoimetric and richer air/fuel ratios, the flame front is generally relatively smooth and somewhat spherical as it progresses outward from the spark plug. However, as the air/fuel ratio is made leaner, the flame front surface loses its smooth character and becomes progressively more ragged. Tongues of flame will occur where the flame finds a more rapidly moving or faster burning mixture ratio. In other regions, where combustion is not as rapid, the flame is retarded behind the more advanced flame tongues. These contour variations arise from chaotic processes and are largely unpredictable. This could explain why the results from ion probe systems employing a single ion probe vary randomly. Sometimes the probe was ionized early by a flame tongue, and sometimes the probe was ionized later by a retarded portion of the flame. One way to compensate for flame front raggedness in learn-burn engines is to sample a large number of combustion chamber cycles with a given probe and then to average the data. However, in most closed-loop control systems sampling a large number of cycles would result in an unacceptably slow response.
The present invention provides a system utilizing ionization probes to accurately detect the flame front, despite flame front raggedness. To accomplish this, several ionization probes are arranged in the combustion chamber roughly equidistant from the spark plug. The electrical current passing through one or more of the probes may then be sensed for use by an engine control system. The initial signal from the array will be caused by the first of the probes to be ionized by the ragged flame front. A more consistent signal may result in this arrangement because the earliest arrival of the flame front at the probe array is consistently detected and also because the multiple electrodes sample a larger portion of the flame front. Thus, a more accurate detection of the flame position may be achieved by using an array of ionization gaps than can be generated by a single ionization gap. Consequently, the variability of the ion probe data can be reduced to the point where the data is of practical use in a closed-loop engine control system.
In the first embodiment of this invention, the multiple ionization probes described above, are located at a position remote but roughly equidistant from the spark plug. At this location, the time from the ignition of the charge in the combustion chamber until the detection of the flame front by the ion probe array is an approximate measure of the charge burning time. This measurement of charge burning time will have reduced variability, so that the charge burning time measured in this way can be used by an engine control system to optimize the engine operation.
In a second exemplary embodiment of the present invention, ionization probes are used to measure ignition delay. Ignition delay is defined as the elapsed time between the ignition spark event and the commencement of a measurable rise in combustion chamber pressure due to buring of the charge. Variability in ignition delay causes changes in the location of peak combustion chamber pressure on the engine crank circle and thus, reduces engine efficiency. Variations in ignition delay are believed to be due, in part, to variations in small-scale turbulence and to small-scale variations in mixture composition in the vicinity of the spark plug gap. Long-term factors may also influence ignition delay. These include the prevailing fuel composition, spark plug gap size and orientation, combustion chamber temperature, and deposits in the intake port and combustion chamber. Long-term factors, which influence ignition delay, may also be partially compensated for with closed-loop control systems, for example, by adjusting the spark advance for a particular combustion chamber of the engine. This embodiment of the present invention measures ignition delay by placing one or more ion gaps in very close proximity to the spark plug. The time duration between the ignition spark event and the arrival of the flame front at the ion gap in close proximity to the spark plug may be considered to be the ignition delay. This measurement of the ignition delay is useful in closed-loop control of the engine.
In a third exemplary system, knowledge of the ignition delay time is used to achieve a more accurate measurement of charge burning time. This is possible because the ignition delay is included in the measurement of the charge burning time in the first embodiment described above. Since the ignition delay varies, a small amount of variation is consequently introduced into the charge burning time calculation. Accordingly, a more accurate charge burning time can be calculated by subtracting a first time duration (measured by the ionization detector close the spark plug) from a second time duration (measured by the ionization detector located remotely from the spark plug). Additionally, if the magnitude of the gross charge burning time measured in this way becomes so short that it indicates the presence of detonation, this information may also be used to retard spark advance. In this way, the system can be used as a knock control system. Finally, because different factors influence ignition delay and the subsequent charge burning time, more cogent control systems may result from an ion probe arrangement which is capable of presenting the control system with separate signals representative of these two combustion parameters.